Method of enriching the oxygen content of air supplied to blast furnaces



g- 1964 M. w. ROBINSON, JR., El'AL METHOD OF ENRICHING THE OXYGEN CONTENT OF,

AIR SUPPLIED TO BLAST FURNACES Filed Nov. 28. 1960 INVEN TOR. Melville W Roblflsoigdr & BY Robert AHiSOfl 4 a BMW A m/TIL,

their ATTORNEYS.

3, nr i United States Patent METHOD OF ENRICHING THE OXYGEN CON- TENT OF AIR SUPPLIED T0 BLAST FURNACES Melville W. Robinson, Jr., Beaver, and Robert J. Allison,

Coraopolis, Pa., assignors to Dravo Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Nov. 28, 1960, Ser. No. 72,004 1 Claim. (Cl. 7541) This invention relates to the field of metallurgy and particularly to the reduction of ore in blast furnaces and like smelting operations where air is employed in the furnace, and is for a method of and apparatus for such purpose.

Our invention has particular application to blast furnaces and will be here particularly described in connection with a blast furnace operation, but the invention is not confined to such use.

It is of course well known that in a blast furnace plant atmospheric air is forced by blowers first through the blast furnace stoves where it is heated and then discharged into the blast furnace. The efl'luent gas from the furnace is cleaned and washed, and is then available for fuel purposes, being largely used to heat the stoves, and for generating steam to drive the blowers for forcing air into the furnace. The air blast of course is constituted of roughly 21% of oxygen and the balance inert gas, principally nitrogen, but with water vapor, carbon dioxide, etc.

In the United States, typical blast furnaces, depending on size, may, for example, operate with from 40,000 to 150,000 cubic feet per minute of air at a gauge pressure of about thirty pounds. The capacity of a blast furnace to produce metal is limited not only by its size, but by various other factors, including among others, the size of the blowers, the size of the stoves, and the gas cleaning and washing facilities. To increase the output of a particular furnace is not merely a matter of supplying more air and more fuel and more ore because of the limited capacity of the accessories to the furnace. Some increase in rate has been achieved by improving the burden, i.e., the character and quality of the material charged into the furnace.

Increase in yield has also been secured by the introduction of commercial oxygen into the blast, but oxygen is so expensive that its use in pig iron production is very restricted, and it is employed only in very special and limited application. If the air is enriched with oxygen to even a few percent (say from the atmospheric content of about 21% to 24 or 25%) the yield may be increased considerably, and since the oxygen then replaces some nitrogen in the total volume of blast supplied to the furnace, there is a reduced consumption of coke because there is less heat yielded to the inert gas to keep the furnace operating and the volume of gases to be heated in the stoves or output gas to be cleaned and Washed is not increased. Hence, while the desirability of using oxygen is known, the cost of oxygen restricts its use.

Elements entering into the cost of oxygen include the substantial capital investment for an oxygen plant, extensive storage facilities, power and labor. Moreover, the physical size of the apparatus is such that few blast furnace plants have space in their immediate area to accommodate such a plant, so that it must be located off the premises, perhaps several thousand feet from the blast furnace plant, and pumped under pressure through a pipe line to the furnace. The oxygen is from 95 to 99% pure, and to produce it from air, the air must be pumped up to 70+ pounds per square inch pressure, while it comes out of the apparatus at just about atmospheric pressure.

3,143,412 Patented Aug. 4, 1964 We have found that it is possible with much less elaborate and expensive and with more compact equipment, and at much lower pressures, to concentrate the oxygen content of air to around 60% oxygen, and according to the present invention, air concentrated in this range is mixed with atmospheric air and the mixture supplied to the blast furnace blower in a ratio of about one volume of the low purity oxygen to about 12 volumes of atmospheric air to produce an oxygen-enriched mixture in the blast carrying about 24% oxygen.

Equipment for production of such low purity oxygen takes little space and can usually be housed alongside the blower house of a blast furnace plant, which house is necessarily close to the stoves and furnaces. Air pressures required for such production need be only of the order of 30 p.s.i.g. Therefore, a blower useful also as a standby blast furnace blower may be used to supply atmospheric air under pressure to the oxygen production unit and may be housed with or adjacent to the blast furnace blower to be attended by the same operators. The low purity oxygen plant may discharge its product directly to the inlet duct of the blast furnace blower, eliminating need for compressing large quantities of low purity oxygen and pushing it through pipe lines many hundred feet long.

As a result of this procedure, the air blast may be enriched with oxygen, but whereas the cost of commercial oxygen has been of the order of $12.00 per ton, we can enrich the blast to the same oxygen content at a cost of about $3.00 per ton. Since an increase in oxygen in the blast of 3% over that contained in the air may reduce the coke consumption by about 40 per ton of pig iron produced, the saving on coke including capital investment and depreciation, may easily offset the cost of the low purity oxygen, thus the capacity of a given furnace may be effectively increased with practically no increased net cost to the furnace owner, or with an overall savings in cost.

We may here point out that users of oxygen for this purpose have usually required oxygen of high purity, or better, and apparatus for economically producing oxygen of about 60% purity has not heretofore been commercially available, so far as we can determine, and such apparatus is now being especially developed for this purpose.

Our invention may be further understood by reference to the accompanying drawings, in which:

FIG. 1 is a schematic view of a blast furnace plant embodying our invention, only so much of the plant as is necessary for an understanding of the present invention being shown; and

FIG. 2 is a similar schematic view of fied arrangement.

Referring first to FIG. 1, 2 designates a blast furnace having a bustle pipe 3 thereabout to which is connected a duct 4 for carrying hot air from the hot blast stoves 5 to the furnace. The stoves 5 are connected to the duct 4 in the usual manner through valves so that when one stove is heating the blast, other stoves may be on a heating-up cycle as is well understood in the art. 6 designates a duct or manifold for supplying air to be heated selectively to any one or more of the several stoves 5. 7 is a pipe through which air is delivered to the manifold 6. The blower house which is always located as close as possible to the blast furnace plant is designated gen erally as A, and is outlined in dot-and-dash lines. Within the blower house is one, and preferably at least two, blowers, these being designated generally as 8 and '8. Their outlets are both connected to the pipe 7 through branch pipes 9. Each blower has an intake pipe 10 leading to an air intake 11. Through the air intake, atmospheric air is drawn through the pipes 10 into the a slightly modia or blowers, then forced through the blast furnace stoves into the blast furnace. This is a usual arrangement. The blowers 8 and 8 are generally driven by steamturbines because of-the availability of blast furnace gas as cheap fuel for generating steam. Two blowers are generally provided, and as above indicated, the air is supplied to the blast furnace up to about 30 p.s.i.g., and depending on the size of the furnace, the output volume may vary between 40,000 and 150,000 cubic feet per minute.

According to the present invention a separate blower or its equivalent may be provided in the blower house. This separate blower is designated 12, and it has an inlet pipe 13 leading to an air inlet 14. The air from the blower 12 is discharged through pipe 15 to a cooler 16 generally provided in oxygen plants for removing the heat of compression. From the cooler 16 the air passes into apparatus within the enclosure designated generally as 17, and which is known in the art of air separation as a cold box. Within this box, as schematically outlined, is an apparatus forming no part of the present invention, wherein a partial separation of oxygen and nitrogen is elfected. According to the present invention the apparatus in the cold box is designed to make only a partial separation. Typically a simple form of apparatus will yield oxygen of a purity of between about 60 and 70%. From the cold box this low purity oxygen is conducted through a pipe 18 to the inlet pipe of one or preferably both of the blowers so as to be drawn into the blowers and discharged into the blast furnace admixed with the atmospheric air. It is desirable to introduce this low purity oxygen to the blower intake, whenever possible. Such apparatus requires minimum ground space and can be. accommodated in an annex designated generally as B for the blower house.

The capacity of the blower 12 and that of the low purity oxygen production apparatus is such that if the oxygen is produced about 60% purity, the low purity oxygen will be about 8% of the total intake for the blowers 8 and 8. This will increase the total oxygen in the blast air to around 24%, as against about 21% contained in the atmospheric air alone.

As pointed out, this additional amount of oxygen will replace an equivalent amount of nitrogen in the blast, thereby reducing the coke requirements of the furnace because of the lesser volume of inert gas to be heated, and it increases the rate of reaction because of the higher concentration of oxygen. The volume of gases discharged from the top of the blast furnace is not materially changed, but the fuel value may be increased because of the decrease in nitrogen. By reason of there being no increase in the output of gas from the furnace, the gas cleaners and washers (not shown) do not need to be a1- tered or increased in capacity in any way.

'With this system the blowers, the oxygen plant and the blast furnace stoves are all close to the blast furnace, and long pipe lines are eliminated and expensive compressor apparatus is not required to convey the oxygen from the oxygen plant to the blowers. The entire output of the oxygen-producing apparatus may be coupled directly into the intake for the blast furnace blowers without requiring large storage facilities, surge tanks and compressors now needed. The oxygen plant itself, because it produces oxygen of about 60% purity, is compact, and since the blower 12 may be located in the blower house along with the blowers 8 and 8', or immediately adjacent to the blower house, the same operators who tend the blast furnace blowing equipment can also operate the oxygen-producing apparatus. The oxygenproducing apparatus may be of a type which operates for several minutes with one set of units for removing water vapor and carbon dioxide from the air, and then which switches automatically to another set of units While the first set' of such units is regenerated, and during this switch-over there may be a short instant, not exceeding pumps a few seconds, when the flow of low-purity oxygen in the line 18 will drop, but this is of no significance since it mean that during that instant additional air will be drawn in through the intake 11 until the supply of oxygen is again restored. I

' If desired, additional piping may be provided whereby the blower 12 may be connected directly to the pipe 9 and disconnected from the oxygen generator only as a temporary expedient when one of the other blowers 8 or 8 has to be taken out of service. Thus the same blower that is accessory to the oxygen plant may also be a standby blower for the blast furnace.

While an important advantage of the present invention is that it eliminates a great part of the equipment and various vessels usedin a high purity oxygen plant, itmay, for certain purposes, be desirable to have a temporary supply of oxygen, should it be necessary to take the oxygen generator out of service. One such method of accomplishing this is disclosed in FIG. 2 where similar reference numerals indicate similar parts, and wherein the plant in all respects is the same as that shown in FIG. 1. However, as is well understood in the operation of oxygen plants, a certain amount of oxygen may be produced as liquid oxygen at the expense of reducing the output of oxygen through the pipe 18. In FIG; 2 we have shown a reservoir 20 connected into the oxygen generator through a pipe 21, and into which some oxygen may be bypassed for storage, a pump being indicated in line 21 at 21a. When the oxygen generator is shut down, a valve indicated at 22 may be opened, allowing the oxygen to flow into an evaporator 23 and from the evaporator through pipe 24 out into the pipe 18. Such an auxiliary storage system is desirable, particularly in the production of ferro-manganese where the failure of low purity oxygen may result in a malfunction of the blast furnace. Such a storage tank need not be of large volume, and can be conveniently located adjacent to the blower house.

Our invention therefore provides a method and apparatus whereby oxygen-enriched air may be used in a blast furnace and other metallurgical processes using an oxidizing blast, at a very low cost per ton of oxygen. The particular components of the apparatus and method are commercially available and constitute no part per se of our invention, and therefore they are only schematically illustrated.

While we have shown and described two specific embodiments of our invention, it will be understood that various changes and modifications may be made therein and it will be further understood that we have mentioned oxygen of about 60% purity only because it is economical and simple to produce the oxygen in this concentration from air in a single stage concentrator, but the invention is applicable to systems where the concentration may be higher or lower. As the concentration of oxygen in the output of the oxygen generator increases, the pressure of the air delivered to the oxygen-generating apparatus must be increased. For 60% oxygen the air input into the oxygen generator need be at a pressure of only about 30 p.s.i.g. and therefore a blower which can also be used as a standby blower for a blast furnace may be used. We have referred to the low purity oxygen as being around 60% because this is the area of concentration that can be economically produced in a single compression-expansion cycle oxygen generator operating at around 30 p.s.i.g. input air pressure. The exact degree of purity is not critical so long as oxygen predominates, but nevertheless a high percentage of nitrogen is still present, and the terms around or about are used in this sense.

The availability of excess blast furnace gas to generate steam for driving the third blower provides especial economy for the use of our invention for blast furnace plants. In other operations waste heat boilers may provide a cheap source of power.

We claim:

The method of enriching the oxygen content of blast air supplied to blast furnaces and other metallurgical furnaces, comprising the steps of,

utilizing adjacent the existing blower supplying atmospheric blast air to such furnace, a second blower, feeding the blast from the second said blower to a cold box type of air separator apparatus adjacent the latter blower capable of supplying oxygen as a liquid and gas, selectively storing said liquid oxygen in an adjacent liquid oxygen reservoir and feeding the gaseous oxygen from said separator apparatus to the atmospheric air inlet pipe of the existing blast blower when said second blower is operating and from the liquid oxygen storage reservoir through a vaporizer to said atmospheric inlet pipe when the said second blower is not feeding blast air to the air separator apparatus, said feeding of the oxygen to said existing blast blower atmospheric air inlet pipe being at pressures above References Cited in the file of this patent UNITED STATES PATENTS Sweetser May 28, 1912 Field Dec. 18, 1928 Karwat May 4, 1937 Reece July 25, 1944 Strassburger Jan. 15, 1957 OTHER REFERENCES Blast Furnace, Coke Oven and Raw Material Committee Proceedings, vol. 6, 1947, pages 50-82. 

